32 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
theory,’’ which forms the framework of nearly all of our modern 
genetics. According to this view the germ plasm is immortal in 
that it is perpetuated from generation to generation through the 
instrumentality of mitotic cell division, each germ cell being the prod- 
uct of the division of a previous germ cell back to the first germ cell 
that arose at the dawn of life. Thus a germ cell cannot be a product 
of the soma, but the soma is the product of germ cells. The soma loses 
its generalized characters and specializes in various ways. Once 
specialized, soma cells are believed to have lost their capacity to play 
a germinal réle. Specialization means mortality. Thus the relation- 
ship between parent and offspring is not that the parent gives rise to 
the offspring, but that the same germ plasm gives rise to both parent 
and offspring. 
The logical conclusion to which this line of reasoning leads is that 
the changes in the soma, no matter how produced, are helpless to 
produce any effect upon the germ plasm, since germ cells come only 
from germ cells and not from soma cells. Consequently Weismann 
led the assault against Lamarckism and won the day so conclusively 
that even in these modern times few biologists have the temerity to 
express aloud any definite belief in the inheritance of acquired charac- 
ters. Weismann’s germ-plasm idea is the cornerstone of modern 
genetics, though there are some forward-looking biologists who, looking 
at things with a physiological bias, cannot make themselves believe in 
the total independence of any tissue—even the sacred germ plasm. 
Weismann’s influence was very great, especially during the last 
decade of the nineteenth century, and his theories gave rise to an 
immense amount of research, chiefly of a cytological and embryo- 
logical character. 
ISOLATION THEORIES 
Among the theories subsidiary to natural selection as an aid to 
species forming are the various isolation theories. One of the weak- 
nesses inherent in natural selection had to do with the probable 
swamping out of new types by promiscuous breeding with the more 
numerous individuals of the older types. “Anything,” says Metcalf, 
“which divides a species into groups, which do not freely interbreed, 
is said to segregate (isolate) the members of the species into these sub- 
divisions.” 
Some American writers, especially Jordan and Kellogg, Gulick, and 
Crampton, have dealt with the isolation factor in evolution and believe 
